Parallel, expanding, through-conduit gate valves are widely utilized in high pressure fluid controlling service such as is typically found in the petroleum industry because the sealing capability thereof can be mechanically controlled to accomplish the necessary seat/gate force for efficient metal-to-metal sealing capability. Moreover, the sealing capability of the valve mechanism can be mechanically controlled both in the open and closed positions thereof through external application of linear force on the expanding gate mechanism. Where gear actuators and hand wheels are employed to accomplish opening and closing movement of the valve mechanism, personnel will simply rotate the hand wheel sufficiently to apply an adequate amount of torque which, through the gear train mechanism, applies desired linear force to the valve stem interconnecting the expanding gate mechanism with the valve actuator. Expanding gate mechanisms take a number of different forms but one widely accepted expanding gate mechanism is representative of the prior art shown generally at 10 in FIG. 1. The parallel expanding gate assembly of FIG. 1 consists of two wedge pieces, namely a gate member 12 and a segment member 14, which are held together by means of arched wire springs 16 having curved extremities 18 and 20 in engagement with pin members 22 and 24 positioned at the upper and lower extremities of the gate member. The intermediate portions of the arched spring wires 16 are positioned in engagement with a pin members 26 which are centrally located on opposite sides of the segment member 14. The force developed by the spring wires 16 on the pins on the gate and segment urge the free segment member toward a fully seated relationship with the gate member so that surfaces 28 and 30 of the segment are disposed in intimate seated, fully engaging relation with both of the angulated surfaces 32 and 34 of the gate member. The angulated surfaces 28 and 30 of the segment and 32 and 34 of the gate are in fact planar cam surfaces which control the position of planar sealing faces 36 and 38 formed respectively on the gate and segment members.
The gate member is movable linearly by means of one or more valve stems 40 and 42. Typically, one of the stems 40 functions as a gate actuating stem while the opposite stem 42 functions as a pressure balancing stem. The valve body structure 44 which defines a valve chamber 46 within which the expanding gate assembly is linearly movable also defines internal stop pads 48 and 50 which are engageable respectively by the upper and lower extremities 52 and 54 of the segment as the segment reaches its limits of travel in either direction. The gate and segment members also define circular port openings shown in broken line at 56 and 58 which become aligned with one another and also aligned with circular flow passages 60 and 62 of the valve body when the expanding gate assembly is in the fully opened and sealed position thereof. The valve body is also formed to define opposed seat recesses 64 and 66 within which seat members 68 and 70 are received. In expanding gate valves it is typical for the seat members 68 and 70 to be press fitted within the seat recesses 64 and 66 so as to establish a nonfloating, interference fitted relationship.
In order to close the valve of FIG. 1, which shows the gate and segment assembly in an intermediate position, a downward force is applied to the gate and segment assembly through the valve stem 40. The gate and segment assembly moves downwardly together until such time as the lower extremity 54 of the segment comes into contact with the stop pad 50. At this time, further downward movement of the segment is prevented by the stop pad while the valve stem continues to move downwardly thereby also causing the gate member to be moved downwardly. This further downward movement of the gate member while the segment member is restrained against downward movement by the stop pad causes relative movement of the angulated planar cam surfaces of the gate and segment. Downward movement of the gate member under this condition causes camming reaction to take place between planar surfaces 28 and 32, thus causing the segment member to be moved transversely to the longitudinal axis of the valve stem. When this occurs, angulated surfaces 30 and 34 become separated and the sealing surfaces 36 and 38 of the gate and segment are moved apart (expanded relative to one another) until they firmly contact the respective sealing faces 72 and 74 of the seat member 68 and 70. The seating force of the gate and segment against the seat members can be increased simply by applying sufficient linear force to the valve stem 40 which, through camming activity of the inclined surfaces, develops sufficient expansion force of the gate and segment against the seat members to provide a proper seal and thus obtain efficient shut off. In the closed position, sufficient sealing force can be developed at the face of the upstream seat 68 by wedging the gate and segments tightly against the seats in the closed position to develop an upstream seal.
Under normal operation, when the stem 40 is moved upwardly to open the valve, the gate also begins to move upwardly. The upstream pressure acting on the segment 14, assisted by the urging force of the arched spring wire 16 causes movement of the segment away from the upstream seat 68, thus moving the segment back into the notch of the gate with the inclined surfaces 28 and 30 of the segment in fully seated contact with the intersecting inclined surfaces 32 and 34 of the gate member as shown in FIG. 1. Further upward movement of the gate and segment assembly occurs together with only the downstream sealing surface 36 of the gate dragging against its respective downstream seat 70. In this condition, the segment member 14 is disposed in spaced relation with the upstream seat 68. With only the downstream sealing surface 36 of the gate in contact with the downstream seat 70, the gate and segment assembly can then be moved to the fully open position with relatively little effort at the hand wheel.